[44.08] A Dynamical Mass Measurement for the Pre-Main-Sequence Secondary of the Eclipsing Binary TY CrA

Using the Danish 50cm telescope at La Silla we have obtained simultaneous
uvby
light curves of the eclipsing binary TY CrA,
located in the Corona Australis star-forming region.
We have securely detected the secondary eclipse (2\% depth in $y$).
We have also obtained high-resolution (R=15000) echelle spectra in the red.
Along with the primary spectrum, absorption lines of the secondary and a
previously unknown tertiary component have been found. In particular,
both the secondary
and tertiary are detected at the Lithium 6708\AA\ line. Based
on temperature insensitive lines the tertiary/secondary luminosity ratio
at $\sim$6400\AA\ is $\sim$1.5.

When combined with our previous single-lined orbital solution for the
primary
(Casey, B.W., Mathieu, R.D., Suntzeff, N.B., Lee, C.W., and Cardelli,
J.A. 1993, Astron. Journal
, 105, 2276)
the secondary radial-velocity measurements
provide a mass ratio of 0.521$\pm$0.007.
Using a modified form of the Wilson-Devinney
formalism, our light curve solution gives an inclination angle of
81\deg, masses and radii of (3.2 M$_\odot$, 1.8 R$_\odot$) and
(1.7 M$_\odot$, 2.3 R$_\odot$)
for the primary and secondary respectively.
Based on both spectral classification and uvby
colors we adopt a primary effective temperature of 12,000 $\pm$ 500 K.
Using Kurucz atmosphere models for both stars in the WD solution, we derive
a
temperature of 5,000 K for the secondary, thus fully specifying
the system.
The primary lies on the ZAMS, while the secondary lies at the base of
the Hayashi tracks. The secondary provides the first dynamical mass
calibration with which to test theoretical calculations of Hayashi tracks.
We will evaluate several modern
theoretical pre-main sequence evolutionary
models with respect to TY CrA.

The $v\sin i$ of the secondary spectrum is 40 km/sec, making the secondary
rotation synchronous with the orbital motion. Given that the primary is
remarkably subsynchronous (Casey et al.
1993 and new spectra),
we conclude that the orbit was tidally circularized by the secondary.